Drainage line units have been available for a number of years. Various U.S. patents describe such units, as well as an apparatus for manufacturing the units, for example, machines for manufacturing drainage lines have been previously described in U.S. Pat. No. 5,154,543. Also, the drainage line units best suited for production by the apparatus of the present invention are as previously described in U.S. Pat. Nos. 5,015,123, 5,051,028, 5,154,543, 5,535,499 (the '499 patent ) and 5,657,527 (the '527 patent).
The previously disclosed and patented apparatus for fabricating the drainage line units, shown in the '527 and '499 patents, relied on machines using mechanical means to push an artificial aggregate material through a mandrel into a sleeve of netting to form the drainage units. The preferred artificial aggregate has been polystyrene or other poly-resinous material, however, as described in the '527 patent, these materials exhibit a high coefficient of friction, which makes it difficult to move this material through a conduit in a manufacturing operation. Mechanical means of pushing the aggregate included a hydraulically operated piston device and a screw drive auger, as shown in FIG. 15 of the '527 patent.
Experience with the manufacturing equipment shown in attached FIGS. 13, 14 and 15 of the '527 has shown that when using polystyrene and similar poly-resinous materials for the aggregate 21, the aggregate exhibits a high degree of surface friction when compacted by operation of the reciprocating hydraulic piston 101. To this end, it has been determined that the direction of compaction is not only longitudinally along the length of the mandrel 91, or conduit, but also that the nature of polystyrene and other poly-resinous materials is such as to expand outwardly into a high friction engagement with the inner surface of the conduit (mandrel 91). This problem occurs no only at the exit end of the mandrel 91, but also at intermediate points along its length, as is depicted in the cross-sectional side view of FIG. 18. In order to solve these problems, it was determined that the application of a low friction coating 250 along the inside surface of the mandrel 91, or conduit, substantially reduces the friction produced by the aggregate material inside the mandrel or conduit. Suitable low friction coatings include a silicone lubricant, or a deposited coating of a low friction polyresin such as nylon or Teflon.RTM. (a trademark of the DuPont Company). The use of the low friction coating dramatically increased the speed at which the hydraulic piston of the prior art could be reciprocated, thereby increasing the productivity of each manufacturing machine.
Rather than being directed at mitigating friction, the present invention overcomes the problem altogether through a new process, thereby resulting in much improved production rates and eliminating the need for lubricants or other means of mitigating friction between the poly-resinous material and the manufacturing equipment.